The present invention relates generally to information coding, and more particularly to techniques for providing channel error protection for source coded bit streams in a wireless communication system or other type of communication system.
Conventional second generation (2G) cellular wireless communication systems, which were designed primarily for voice services, generally do not provide a sufficiently low bit error rate (BER) and frame error rate (FER) to support high-quality audio, image or video transmission. The typical BER and FER provided in such systems is on the order of 10xe2x88x923 and 10xe2x88x921, respectively, as measured at the output of a channel decoder. While satisfactory voice quality can be obtained in the presence of these error rates, the rates are generally too high to allow acceptable audio, image or video quality to be obtained. By way of example, a BER of less than about 10xe2x88x925 at the channel decoder output is considered desirable for digital music and video transmissions.
The bits in a given source coded bit stream (e.g., a compressed audio, image or video bit stream) often have different levels of importance in terms of their impact on reconstructed signal quality. As a result, it is generally desirable to provide different levels of channel error protection for different portions of the source coded bit stream.
Techniques for use in providing such unequal error protection (UEP) through the use of different channel codes are described in U.S. patent application Ser. No. 09/022,114, filed Feb. 11, 1998 in the name of inventors D. Sinha and C. -E. W. Sundberg and entitled xe2x80x9cUnequal Error Protection for Perceptual Audio Coders,xe2x80x9d and subsequently issued on Jun. 11, 2002 as U.S. Pat. No. 6,405,338, which is incorporated by reference herein. In one such technique, a source coded bit stream is divided into different classes of bits, with different levels of error protection being provided for the different classes of bits. For example, a source coded bit stream may be divided into class I and class II bits, with class I bits protected by a lower rate channel code than that used to protect the class II bits.
So-called third generation (3G) cellular wireless communication systems currently under development have a target BER of about 10xe2x88x925 at the channel decoder output. These 3G systems are thus being specifically designed to support the transmission of compressed audio, image and video bit streams. However, a significant problem with the 3G systems is that such systems are generally not being configured to support UEP for different portions of a given source coded bit stream. Although channel code approaches such as those described in the above-cited U.S. patent application Ser. No. 09/022,114 may be used to provide UEP in a 3G wireless system, a physical layer of the wireless system will need to be supplied with information characterizing the particular nature of the source coded bit stream it is transmitting before it can apply the appropriate channel protection to different portions of that bit stream. Incorporating a mechanism for providing the physical layer of the wireless system with information regarding the source coded bit stream will generally require some alteration in the physical layer infrastructure of the system, and is therefore undesirable.
It is apparent from the foregoing that a need exists for improved channel error protection techniques for the transmission of source coded bit streams, such as compressed audio, image or video bit streams, in 2G and 3G cellular wireless systems as well as other types of communication systems.
The present invention meets the above-described need by providing a method and apparatus for implementing channel error protection for a source coded bit stream in a wireless communication system in a manner which does not require alteration of physical layer infrastructure of the system.
In accordance with one aspect of the present invention, channel error protection is provided for a source coded bit stream in a communication system by a combination of outer channel coding and inner channel coding implemented across different network layers of the system. One or more of a number of different portions of the source coded bit stream are outer channel coded in a first network layer of the system, e.g., an application layer, using a designated outer channel code, e.g., a Reed Solomon (RS) code or other type of block code, so as to provide an outer channel coded bit stream having different levels of error protection for each of the different portions of the source coded bit stream. The different portions may correspond to designated portions of each of one or more frames of the source coded bit stream. The outer channel coded bit stream is then inner channel coded in a second network layer of the system, e.g., a physical layer, using a designated inner channel code, e.g., a convolutional code or a turbo code, to thereby generate a channel coded bit stream. The channel coded bit stream may then be subject to further processing operations prior to transmission in the communication system.
In accordance with another aspect of the present invention, code words of the outer channel coded bit stream associated with a given one of the portions of the source coded bit stream may be arranged in different modem frames of the communication system prior to application of the inner channel code.
In accordance with a further aspect of the present invention, the outer channel coding may be applied adaptively based at least in part on feedback from a receiver of the system.
Advantageously, the invention can provide sufficiently low error rates in a 2G cellular wireless system such that high-quality audio, image, video or multimedia transmission can be supported. In addition, the invention allows UEP to be provided in a standard 2G or 3G system in an efficient manner without modification of physical layer system infrastructure. Although particularly well-suited for use with progressive source coded video bit streams, the invention is more generally applicable to any type of prioritized information bit stream requiring channel error protection, including partitioned source coded bit streams and scalable, layered source coded bit streams.